Discontinuous conductive filler polymer-matrix composites for electromagnetic shielding

ABSTRACT

A medical electrical lead having a conductor assembly covered by an insulating layer, and a shield covering positioned adjacent or proximate to at least a portion of the insulating layer in order to shield the conductor assembly from one or more electromagnetic fields. The shield covering is formed of a polymer-matrix composite. The polymer-matrix composite includes a polymeric resin having discontinuous conductive fillers provided therein. The discontinuous conductive fillers include one or more of nano-sized metal structures and nano-sized non-metallic conductive structures. The nano-sized non-metallic conductive structures can have a coating formed of one or more metals. The nano-sized non-metallic conductive structures can be formed of carbon. In turn, the nano-sized non-metallic conductive structures can include one or more of carbon nanofibers, carbon filaments, carbon nanotubes, and carbon nanoflakes.

FIELD

The present invention relates generally to medical devices, and, moreparticularly, to reducing the effects of electromagnetic radiation onsuch medical devices.

BACKGROUND

Since their earliest inception, implantable medical devices (IMDs) havecontinually been advanced in significant ways. Today, IMDs includetherapeutic and diagnostic devices, such as pacemakers,cardioverter/defibrillators, hemodynamic monitors, neurostimulators, anddrug administering devices, as well as other devices for alleviating theadverse effects of various health ailments.

As is known, modern electrical therapeutic and diagnostic devices forthe heart and other areas of the body generally include an electricalconnection between the device and the body. This connection is usuallyprovided by at least one medical electrical lead. For example, aneurostimulator delivers mild electrical impulses to neural tissue usingone or more electrical leads. In turn, such neurostimulation oftenresults in effective pain relief and a reduction in the use of painmedications and/or repeat surgeries. Each electrical lead used with suchdevices typically takes the form of a long, generally straight,flexible, insulated conductor. At its proximal end, the lead istypically connected to a connector of the device, which also may beimplanted within the patient's body. Generally, one or more electrodesare located at or near the distal end of the lead and are attached to,or otherwise come in contact with, the body. Such devices may becontrolled by a physician or a patient through the use of an externalprogrammer.

It is well known that, if not shielded sufficiently, the implanted leadsof medical devices can be adversely affected when a patient is exposedto alternating electromagnetic fields. Alternating electromagneticfields can generally stem from any of a number of radio-frequencyradiation sources, e.g., magnetic resonance imaging (MRI) systems asdescribed below. As such, if an implanted medical lead is notsufficiently shielded, electromagnetic fields can induce an electriccurrent within a conductor of the lead. In turn, such an implantedelectrical lead would act as an antenna, resulting in an electricalcurrent that flows from the electrode of the lead and through bodytissue. Because the tissue area associated with electrode contact may bevery small, the current densities may be high, which can result intissue heating that may cause damage.

There can be other limitations associated with exposing implanted leadsof medical devices to electromagnetic fields and/or radio-frequencyenergy if the leads are not sufficiently shielded therefrom. As isknown, a sudden burst of radio-frequency energy can cause an electricpulse within the lead. The medical device, as should be appreciated, cansense the imposed voltage on the lead, and in turn, may cause the deviceto respond inappropriately, resulting in the wrong therapy beingadministered to the patient at that time or in the future. For example,with respect to cardiac IMDs, inappropriate therapy modification may beone response of the IMD, which can involve changing the rate orthresholds associated with pacing pulses.

As is known, magnetic resonance imaging (MRI) is an anatomical imagingtool which utilizes non-ionizing radiation (i.e., no x-rays or gammarays) and provides a non-invasive method for the examination of internalstructure and function. For example, MRI permits the study of theoverall function of the heart in three dimensions significantly betterthan any other imaging method. Furthermore, MRI scanning is widely usedin the diagnosis of diseases and injuries to the head. Magneticresonance spectroscopic imaging (MRSI) systems are also known and areherein intended to be included within the terminology “MRI” systems orscanners. These MRI systems can be used to give valuable diagnosticinformation, but also subject the patient to significant alternatingelectromagnetic fields and/or radio-frequency energy, which may resultin one or more of the undesirable effects described above with respectto IMDs or medical devices using implanted leads.

A variety of different coverings have been used for implantable leads ofmedical devices to overcome the above limitations. Some coverings haveinvolved metal or metal alloy wire being braided around the lead,thereby forming a shield having a large conductive surface area. Othercoverings have involved the use of polymer-matrix composites. Suchcomposite coverings, as opposed to metal wire coverings, are attractivedue to their moldability. In addition, the composite coverings are morefavorable because metal wire coverings can be prone to damage byscratching, abrasion, or wear.

Polymer-matrix composite coverings are conductive due to the presencetherein of electrically conducting fillers, which can be discontinuous(e.g., such as particles or short fibers) or continuous (e.g., such ascontinuous fibers). As is known, even though they lack the continuityprovided by continuous fillers, discontinuous fillers can just as wellbe used for electromagnetic shielding. Moreover, discontinuous fillersare suitable for composite fabrication by extrusion or injection moldingand, if the discontinuous filler is fine enough in size, even by ink-jetprinting or screen printing. Due to the lower cost and greaterversatility of composite fabrication for discontinuous fillers comparedto continuous fillers, discontinuous fillers have been widely used inmaking electrically conducting composites, especially those forelectromagnetic shielding.

While polymer-matrix composites having discontinuous fillers have beenused as lead coverings to reduce the effects of electromagneticradiation, they have been found to present certain limitations, e.g.,with respect to their overall shielding effectiveness. What is needed isapparatus used to reduce the potential adverse effects to medicaldevices, and in particular, to implantable electrical leads of thedevices, when subjected to electromagnetic radiation, while furtherovercoming one or more of the limitations facing the discontinuousfiller polymer-matrix composite lead coverings marketed to date.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention relate to a medical electrical lead havinga conductor assembly covered by an insulating layer, and a shieldcovering positioned adjacent or proximate to at least a portion of theinsulating layer in order to shield the conductor assembly from one ormore electromagnetic fields. The shield covering is formed of apolymer-matrix composite. The polymer-matrix composite includes apolymeric resin having discontinuous conductive fillers providedtherein. The discontinuous conductive fillers include one or more ofnano-sized metal structures and nano-sized non-metallic conductivestructures. The nano-sized non-metallic conductive structures can have acoating formed of one or more metals. The nano-sized non-metallicconductive structures can be formed of carbon. In turn, the nano-sizednon-metallic conductive structures can include one or more of carbonnanofibers, carbon filaments, carbon nanotubes, and carbon nanoflakes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary IMD as provided in apatient in accordance with certain embodiments of the invention.

FIG. 2 is a perspective view of another exemplary IMD as provided in apatient in accordance with certain embodiments of the invention.

FIG. 3 is a perspective view of a further exemplary IMD as provided in apatient in accordance with certain embodiments of the invention.

FIG. 4 is a plot generally showing skin depth for carbon and metals athigh frequencies of electromagnetic fields.

FIG. 5 is a plot generally showing conductivity of carbon nanofibers andcarbon nanotubes at varying weight percentages of polymer-matrixcomposites.

FIG. 6 is a plot generally showing conductivity of carbon nanofibers atvarying volume percentages of polymer-matrix composites.

FIG. 7 is a plot generally showing electromagnetic shieldingeffectiveness of carbon micro fiber, carbon filament, and nickelfilament.

FIG. 8 is a perspective view of a partial implantable lead/extensionhaving a shield covering in accordance with certain embodiments of theinvention.

FIG. 9 is a cross-sectional view of an implantable lead/extension havinga plurality of shield coverings, including those of FIG. 8, inaccordance with certain embodiments of the invention.

FIG. 10 is a cross-sectional view of an implantable lead/extensionhaving another plurality of shield coverings, including those of FIG. 8,in accordance with certain embodiments of the invention.

FIG. 11 is a cross-sectional view of an implantable lead/extensionhaving a further plurality of shield coverings, including those of FIGS.8, 9, and 10, in accordance with certain embodiments of the invention.

FIG. 12 is a cross-sectional view of an implantable lead/extensionhaving another further plurality of shield coverings, including thoseFIG. 8, in accordance with certain embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description should be read with reference to thedrawings, in which like elements in different drawings are numberedidentically. The drawings depict selected embodiments and are notintended to limit the scope of the invention. It will be understood thatembodiments shown in the drawings and described below are merely forillustrative purposes, and are not intended to limit the scope of theinvention as defined in the claims.

Embodiments of the invention relate to medical devices, and specificallyrelate to shield coverings for leads extending between the devices andthe patient. Embodiments described and illustrated herein pertain toimplantable medical devices (IMDs); however, the invention can extend toany lead-bearing medical device, whether implantable or not.Furthermore, while the embodiments provided herein relate to certainIMDs, it should be appreciated that such embodiments are exemplary innature. As such, the invention is not limited to any particular IMD, butinstead is applicable to any IMD, including therapeutic and diagnosticdevices, such as pacemakers, cardioverter/defibrillators, hemodynamicmonitors, neurostimulators, and drug administering devices, as well asother devices for alleviating the adverse effects of various healthailments.

FIG. 1 illustrates an exemplary IMD in accordance with certainembodiments of the invention. The IMD 10 shown is a typical spinal cordstimulation (SCS) system and includes a pulse generator such as a SCSneurostimulator 12, a lead extension 14 having a proximal end coupled tothe neurostimulator 12, and a lead 16 having a proximal end coupled to adistal end of the extension 14 and having a distal end coupled to one ormore electrodes 18. The neurostimulator 12 is typically placed in theabdomen of a patient 20, and the lead 18 is placed somewhere along thepatient's spinal cord 22. While only shown with a single lead 18, it isto be appreciated that the IMD 10, in certain embodiments, can have aplurality of leads. Such a system may also include a physicianprogrammer and a patient programmer (not shown).

The neurostimulator 12 may be considered to be an implantable pulsegenerator and capable of generating multiple pulses occurring eithersimultaneously or one pulse shifting in time with respect to the other,and having independently varying amplitudes and pulse widths. Theneurostimulator 12 contains a power source and electronics for sendingprecise, electrical pulses to the spinal cord 22 to provide the desiredtreatment therapy. While the neurostimulator 12 typically provideselectrical stimulation by way of pulses, other forms of stimulation maybe used such as continuous electrical stimulation.

The lead 16 includes one or more insulated electrical conductors eachcoupled at their proximal end to a connector 24 and to the electrodes 18(or contacts) at its distal end. As is known, some leads are designed tobe inserted into a patient percutaneously and some are designed to besurgically implanted. In certain embodiments, the lead 16 may contain apaddle at its distant end for housing the electrodes 18. In alternateembodiments, the electrodes 20 may comprise one or more ring contacts atthe distal end of the lead 16.

While the lead 16 is shown as being implanted in position to stimulate aspecific site in the spinal cord 22, it could also be positioned alongthe peripheral nerve or adjacent neural tissue ganglia or may bepositioned to stimulate muscle tissue. Furthermore, electrodes 18 (orcontacts) may be epidural, intrathecal or placed into spinal cord 22itself. Effective spinal cord stimulation may be achieved by any ofthese lead placements. While the lead connector at proximal end of thelead 16 may be coupled directly to the neurostimulator 12, the leadconnector is typically coupled to the lead extension 14 as is shown inFIG. 1.

FIG. 2 illustrates another exemplary IMD in accordance with certainembodiments of the invention. The IMD 30 shown is a typical deep brainstimulation (DBS) system spinal cord stimulation (SCS) system andincludes substantially the same components as does an SCS; that is, atleast one neurostimulator, at least one extension, and at least onestimulation lead containing one or more electrodes. As can be seen, eachneurostimulator 32 a and 32 b is implanted in the pectoral region ofpatient 34. Corresponding extensions 36 a and 36 b are deployed upthrough the patient's neck, and corresponding leads 38 a and 38 b areimplanted in the patient's brain 40 as is shown at 42 a and 42 b. As canbe seen, each of the leads 38 is connected to its respective extension36 just above the ear on both sides of the patient 34.

FIG. 3 illustrates a further exemplary IMD in accordance with certainembodiments of the invention. The IMD 50 is a cardiac medical device,exemplarily shown as a pacemaker, and includes one or more leads 52implanted in a patient 54. The leads 52 extend from the pacemaker can 56and lead into the patient's heart 58 via a vein 60. Located generallynear distal ends 62 of the leads 52 are one or more exposed conductiveelectrodes 64 that are attached to the heart tissue for sensing cardiacactivity, delivering electrical pacing stimuli, and/or providing acardioversion/defibrillation shock to the heart 58. The contact areabetween the electrodes 64 and the tissue of the heart 58 may be verysmall as compared, for example, to the contact area between the IMD 50and the patient's body.

Implantable leads of IMDs similar to those described above in FIGS. 1-3,as well as implantable leads of other medical devices, have beenequipped to reduce the effect from electromagnetic fields and/orradio-frequency energy, e.g., which can stem from MRI systems. To thisend, the leads can be covered with polymer-matrix composites.Polymer-matrix composites, as used herein, include a polymeric resinhaving conductive fillers provided therein. As described above, thefillers can be discontinuous or continuous. As further described above,polymer-matrix composites having discontinuous fillers tend to beparticularly attractive, as opposed to continuous filler composites, dueto their lower cost and greater versatility of fabrication.Unfortunately, polymer-matrix lead coverings having discontinuousfillers have been found to present challenges, e.g., with respect totheir overall shielding effectiveness.

One material that can be used when forming discontinuous fillers for usein polymer-matrix composites is carbon; however, the invention shouldnot be limited to such. Instead, any other conductive materialdemonstrating similar advantageous properties, as described herein withrespect to carbon, may be alternatively used. When provided in adiscontinuous form, carbon can be presented in a wide variety ofstructures, including particulates, powders, fibers, filaments, flakes,and the like. In general, such structures are commercially available andpractical to produce in sizes typically on the order of micrometers. Forexample, carbon filaments/fibers can be provided having outer diametersranging in size from about 0.1 μm to about 12 μm. As should beappreciated, these diameter sizes are significantly reduced from thediameter sizes of commercially available metal wire (e.g., typicallyhaving minimum outer diameters of about 20 μm). As a result, suchdiscontinuous carbon fillers are found to be less susceptible to theskin effect at high frequencies as compared to discontinuous metal wirefillers, as described below.

In general, when a static electrical field is applied to a conductor,the mobile charges therein, e.g., the electrons, are found to move andcreate a direct current (DC), which is uniformly distributed on theentire cross section of the conductor, resulting in a uniform currentdensity. However, when an electromagnetic field is imposed on such aconductor, the mobile charges therein are found to oscillate back andforth with the same frequency as the impinging fields. The movement ofthese charges constitutes an alternating current (AC). Due to theattenuation of the electromagnetic waves in the conductor, theconductor's current density is greatest at the conductor's surface anddeclines exponentially as a function of depth. The decline in currentdensity versus depth is known as the skin effect, and the skin depth isa measure of the distance over which the current falls from its value atthe conductor's surface.

FIG. 4 is a plot illustrating skin effect for carbon and a number ofmetals, illustrating the relationship described above. Specifically, theplot shows skin depth (in μm) for carbon and a number of metal wires athigh frequencies (in GHz) of electromagnetic fields. As illustrated,skin depth for the materials generally decreases as frequency increases(with respect to electromagnetic fields surrounding the materials).Thus, while metals are generally known to exhibit high conductivity, thecross-sectional area of these metals which is actually usable for suchpurposes is limited due to the skin effect.

As shown, carbon is also limited due to the skin effect, but to a lesserextent than the metals. However, as described above, carbon fiber can bemade having a smaller diameter than metal wire. As a result, a largercross-sectional area of the carbon fiber can be used for conductivepurposes in comparison to conventionally available metal wire.Therefore, carbon fiber can be found to be more efficient than metalwire, in particular, when used as a shield in the presence of highfrequency electromagnetic fields. For example, as demonstrated in FIG.4, skin depth of conductive metals is typically less than 1 μm when thefrequency of surrounding electromagnetic fields is about 64 MHz orhigher. As such, discontinuous metal fillers, even having minimumdiameters of about 20 μm, are not very effective shields because much oftheir cross-sectional area would carry very little current. In contrast,discontinuous carbon fibers, which can be provided having outerdiameters of about 0.1 μm to about 12 μm can be much more effective intheir use as shields because a higher percentage of their cross sectioncan be used for carrying current, and in turn, conducting.

As is known, for a polymer-matrix composite, the electromagneticradiation shielding effectiveness increases with increasing fillervolume fraction in the composite. However, the maximum filler volumefraction is limited by the poor composite mechanical properties at highfiller volume fractions resulting from the poor filler-matrix bonding.As described above, discontinuous carbon can generally be preferable todiscontinuous metal wire because a higher percentage of thecross-sectional area of the carbon can be used for conductivitypurposes, particularly in the presence of high frequency electromagneticfields. Though, even with this increased efficiency, the usablecross-sectional area of the carbon is generally found to be lessconductive than metal wire. However, because the carbon, e.g., carbonfiber, is smaller in diameter than metal wire, a greater quantity ofsuch carbon can be utilized in the matrix material. In turn, theincreased efficiency of the carbon (as described above with respect toskin depth) with the greater quantity of carbon in the matrix materialenables the carbon to collectively exhibit enhanced conductivitythroughout the composite. As such, using micro-sized carbon inpolymer-matrix composites provides a more effective shield as a leadcovering in the presence of electromagnetic fields as opposed to metalwire.

Further, when fillers are provided that are conductive yet smaller insize than their alternatives, as is the case in the above examplecomparing micro-sized carbon and metal wire, the matrix material of thecomposite can also be reduced while still maintaining the warrantedvolume fractions of the composite. As a result, a thinner lead coveringcan be provided that is more effective at shielding the electrical lead.Further, provided the micro-sized carbon and other alternative fillermaterial are comparable in price, materials and process savings can berealized by using micro-sized carbon while still providing goodmechanical properties for the composite.

While the above embodiment demonstrates higher electromagnetic radiationshielding effectiveness being achieved when using micro-sized carboninstead of metal wire as discontinuous fillers in polymer-matrixcomposites, even higher shielding effectiveness can be achieved in suchcomposites. As described above, carbon structures can be madediscontinuous so as to have sizes on the order of micrometers. However,carbon structures can also be further reduced in size on the order ofnanometers. Examples of such carbon structures include nanofibers,filaments, nanotubes, and nanoflakes. Alternatively, certain metals canbe formed into nano-sized structures, e.g., particles and/or flakes,which would also be more effective as fillers in comparison todiscontinuous wire or micro-sized carbon structures. Such fillers caninclude but are not limited to Ag, Au, Cu, Co, Ni, Pt, Sn, Ta, Ti, Zn,or any alloys thereof.

Accordingly, in certain embodiments, a polymer-matrix composite isprovided having discontinuous fillers, with the fillers including one ormore of nano-sized non-metallic conductive structures and nano-sizedmetal structures. The use of the term “structures” herein with respectto “nano-sized non-metallic conductive structures” and “nano-sized metalstructures” is meant to indicate “one or more different structures” inmaking up the fillers in each case. In certain embodiments, as describedabove, the nano-sized non-metallic conductive structures can be formedof carbon. In turn, in certain embodiments, the nano-sized non-metallicconductive structures can include one or more of carbon nanofibers,carbon filaments, carbon nanotubes, and carbon nanoflakes. For example,carbon nanofibers are widely available from a number of commercialsources, one being Pyrograf Products, Inc., an affiliate of AppliedSciences, Inc., located in Cedarville, Ohio. One such carbon nanofiberprovided by Pyrograf Products, Inc. is the Pyrograf®-III, ranging indiameter from about 70 and about 200 nanometers. In many cases, carbonnanofiber is commercially available ranging in diameter from about 70 toabout 500 nanometers. The nano-sized metal structures can be introducedto the composite using commercially available nano-sized metal particlesor by means of in-situ reduction of desired metal ions injected into thematrix via chemical method, metal ion implantation method, or othersuitable methods.

As should be appreciated, similar to that already described above incomparing discontinuous fillers of conventional carbon structures andmetal wire, when nano-sized carbon and/or nano-sized metal are usedinstead of the conventional micro-sized carbon and/or metals in formingthe fillers, the resulting composites would have enhancedelectromagnetic shielding effectiveness. As such, discontinuous fillersformed of nano-sized carbon structures and/or nano-sized metalstructures, because of their reduced size (as compared to conventionalmicro-sized carbon and/or metals of same volume fraction), would be lesssusceptible to skin effect at high frequencies. Further, because oftheir reduced size, the discontinuous fillers formed of nano-sizedcarbon structures and/or nano-sized metal structures can be provided inincreased quantity in the matrix material compared to micro-sized carbonfiber structures and discontinuous metal wire.

In some cases, nano-sized carbon structures may be selected overnano-sized metal structures. One reason may be due to costconsiderations. However, as should be appreciated, using nano-sizedcarbon structures in polymer-matrix composites can be costly as comparedto using conventional micro-sized carbon structures. One way to addressthis is to select the least costly of the carbon nano-sized structures.As is known, carbon nanofibers and carbon nanoflakes are less expensiveto produce than carbon nanotubes. For example, material cost to produceone pound of Pyrograf®-III carbon nanofibers is less than $50, whileproducing the same amount of single walled carbon nanotubes, e.g., fromCarboLex, Inc., located in Lexington, Ky., is about $16,030. As such, interms of cost considerations, carbon nanofiber is generally preferredover carbon nanotubes.

Further advantages in using carbon nanofiber are illustrated withrespect to FIGS. 5 and 6. FIG. 5 is a plot illustrating electromagneticshielding effect of carbon nanofibers and carbon nanotubes.Specifically, the plot shows bulk conductivity (in S/cm) for carbonnanofibers (from Pyrograf Products, Inc.) and single walled carbonnanotubes (from CarboLex, Inc.), given varying weight percentages of thecarbon materials in polymer-matrix composites. As shown, the minimumrequirement for electromagnetic shielding (about 1 S/cm) is initiallymet for carbon nanofibers when provided at about 10% weight of thecomposite, and then exceeded given greater weight percentages. Incontrast, the minimum requirement for electromagnetic shielding isbarely met by the carbon nanotubes, and only when the carbon nanotubesare provided at between about 25% and 30% weight of the composite. FIG.6 is a plot illustrating conductivity (in S/cm) for carbon nanofibers,given varying volume percentages in polymer-matrix composites. As shown,the curve is similar to the one shown in FIG. 5 for carbon nanofibers,where the minimum requirement for electromagnetic shielding (about 1S/cm) is initially met for the carbon nanofibers when provided at about7% weight of the composite, and then exceeded given greater weightpercentages. In summary, carbon nanofibers demonstrate significantlyhigher electromagnetic shielding effectiveness as compared to carbonnanotubes.

One challenge encountered to date in using nano-sized carbon structuresas discontinuous fillers in polymer-matrix composites is their naturaltendency to clump together. As such, effective dispersion of thenano-sized carbon structures throughout the matrix material can bedifficult, which in turn, can compromise the shielding effectiveness ofsuch composites. For example, if a quantity of carbon nanofibers isselected based on a uniform distribution throughout the matrix material,if the fibers clump, this uniform distribution would be difficult toachieve. In turn, the shielding effectiveness would likely not beuniformly dispersed through the composite, leaving some areas moresusceptible to penetration by electromagnetic radiation.

It has been found through experimentation that this clumping phenomenonof nano-sized carbon structures can be overcome. Specifically, byproviding a thin coating of metal on the nano-sized carbon structuresprior to their disbursement in matrix material when formingpolymer-matrix composites, one can offset the natural tendency of thenano-sized carbon structures to clump together. As a result, uniformshielding effectiveness for the composite can be achieved usingnano-sized carbon structures, e.g., carbon nanofibers, which have atendency to clump together. As such, in certain embodiments, carbonnanofibers, carbon filaments, carbon nanotubes, and carbon nanoflakesused as fillers can be provided with a metal coating. Such metalcoating, in certain embodiments, can be provided so as to be no greaterthan about 1 μm. Metalization of carbon nanofibers, carbon filaments,carbon nanotubes, and carbon nanoflakes can be achieved via physicalvapor deposition, chemical vapor deposition, auto-catalytic electrolessdeposition, or other metallization techniques known to the art. Themetal coating can include but are not limited to Ag, Au, Cu, Co, Ni, Pt,Sn, Ta, Ti, Zn, alloys thereof, as well as any combination thereof.

It should be appreciated that such metal coating also enhances theconductivity, and thus, shielding effectiveness, of the carbonstructures. For example, the metal coating can be made of nickel. FIG. 7shows electromagnetic shielding effectiveness of carbon fiber, carbonfilament, and nickel filament, each at commercially available diametersizes. Of the three, the nickel filament demonstrates the highestelectromagnetic shielding effectiveness, where the volume fraction ofthe nickel filament indicates that such composite can be processed,e.g., as tubing, to be used as a coating for discontinuous fillers inpolymer-matrix composites.

FIG. 8 illustrates a perspective view of a partial implantablelead/extension having a shield covering in accordance with certainembodiments of the invention. As shown, the lead/extension 70 includes aconductor assembly having one of more conductors (not shown) covered byan insulating layer 72. As should be appreciated, the one or moreconductors can be formed of any suitable metal having conductiveproperties, such as Cu, Al, Ag, alloys thereof or other alloys (e.g.,superalloy MP35N), mixtures thereof, and the like. Further, as should beappreciated, each of the one or more conductors can be configured intoone of a wide variety of shapes, e.g., generally straight, helicallywound, etc. The insulating layer 72 is generally formed of silicone, abiocompatible polymer such as polyurethane, or any suitablebiocompatible, non-conducting material known in the art.

As shown, the lead/extension 70 has a shield covering 74, whichfunctions to shield electromagnetic radiation from the lead conductors.In certain embodiments, the shield covering 74 is provided as apolymer-matrix composite. As described herein, the polymer-matrixcomposite generally includes a polymeric resin 76 having conductivefillers provided therein. As shown, in certain embodiments, theconductive fillers are discontinuous conductive fillers. Thediscontinuous conductive fillers, in certain embodiments, can benano-sized non-metallic conductive structures 78, nano-sized metalstructures 80, or the combination of both (as exemplarily shown in FIG.8). The distribution of the fillers can be uniformly distributedthroughout the composite matrix (as illustrated in FIG. 8), orgradiently distributed with higher concentration at the surface of theshield tube (not shown in the Figure) so as to take advantage of bothshield effect and mechanical properties of the polymer composite tube.The polymeric resin may be selected from any of a wide variety ofthermoplastic resins and elastomers, blends of thermoplastic resins,and/or thermoset resins. Some nonlimiting examples of each of theseresin types are provided in U.S. patent application Ser. No. 09/683,069,entitled “Conductive Plastic Compositions and Method of ManufactureThereof”, incorporated herein in relevant part.

As described above, in certain embodiments, the nano-sized non-metallicconductive structures 78 can be formed of carbon. The carbon structures,in certain embodiments, can include one or more of carbon nanofibers,carbon filaments, carbon nanotubes, and carbon nanoflakes. As should beappreciated, when the nano-sized carbon structures are selected, onewould need to select the particular grade for the structures. Forexample, as is known, there are many grades of carbon fiberscommercially available. As such, one would select the grade which has oris able to be produced to have a desirable diameter dimension, yet alsoexhibits low electrical resistivity and high strength properties. Incertain embodiments, the fillers have an outer diameter ranging in sizefrom about 70 nanometers to about 500 nanometers.

As further shown, in certain embodiments, the nano-sized non-metallicconductive structures 78 are each coated with a metal coating 82,including but not limited to Ag, Au, Cu, Co, Ni, Pt, Sn, Ta, Ti, Zn,alloys thereof, as well as any combination thereof. As such, thenano-sized non-metallic conductive structures 78 can be prevented fromclumping so as to be uniformly disbursed in the resin 76 and theconductivity of the fillers can be enhanced. The covering 74 is not indirect electrical contact with the conductors of the conductor assemblyof the lead/extension 70. In certain embodiments, the covering 74 can bein contact with the housing of the medical device (e.g., IMD) from whichthey stem, where the device can act as an additional surface fordissipation of energy received by the covering 74 from electromagneticradiation.

It should be appreciated that the lead/extension 70 can be either alead, an extension for a lead, or both. For example, with respect to theIMD 10 of FIG. 1, the leads 16, the lead extensions 14, or both, can beequipped with the covering 74. The above similarly holds true for theleads 38 a, 38 b and extensions 36 a, 36 b with respect to the IMD 30 ofFIG. 2.

The amount of the discontinuous fillers, or combinations of fillersformed of different materials, to be incorporated into the matrixmaterial can vary depending on the desired properties exhibited by theparticular medical device or medical device component. As describedabove, enough of the discontinuous fillers should be included so thatdesired properties are at least minimally exhibited by the composite,but not so much of the fillers should be included so as to have adetrimental effect on the properties of the composite. While theparticular range may vary depending on the filler and matrix materialsbeing utilized, composites exhibiting advantageous properties can beobtained by incorporating from about 0.005% to about 99% filler materialrelative of the total final composition weight of the composite. In manyembodiments, filler material may be incorporated in an amount of fromabout 0.01% up to about 40% or 50% by weight of the composite. In atypical embodiment, the filler material can be incorporated in an amountof from about 0.1% to about 20% of the composite, for example, fromabout 1% to about 10% by weight of the composite.

Each of FIGS. 9-12 shows a cross-sectional view of an implantable leadhaving a plurality of shielding coverings in accordance with certainembodiments of the invention. Each of the implantable leads/extensionsof FIGS. 9-12 includes the insulating layer 72 and the covering 74 shownand described in FIG. 8. Further, each of the leads/extensions 70 ofFIGS. 9-12 includes one or more additional layers located proximate oradjacent to the covering 74. While each of the insulating layers 72, thecoverings 74, and the one or more additional layers are shown to havesimilar thicknesses in FIGS. 9-12, such is done to merely show theproper position of the layers with respect to one another. As such, theinvention should not be limited by the thickness of the layersrepresented in FIGS. 9-12.

FIG. 9 illustrates lead/extension 90. In certain embodiments, as shown,a coating 92 lies external to the covering 74. In certain embodiments,the coating 92 comprises one or more metals. As such, the coating 92 isadapted to enhance the shielding effect of the lead/extension 90. Incertain embodiments, the one or more metals can include Ag, Au, Cu, Co,Ni, Pt, Sn, Ta, Ti, Zn, alloys thereof, as well as any combinationthereof; however, the invention should not be limited to such. Instead,the one or more metals can include any metal or combination of metalswhich can be used in conjunction with the covering 74 to enhance theshielding effect of the lead/extension 90. In certain embodiments, ifthe coating 92 is to be kept thin, the coating 92 may be applied to thecovering 74 via sputtering, physical vapor deposition, chemical vapordeposition, auto-catalytic electroless deposition, electrolyticdeposition, or by any other suitable application method.

FIG. 10 shows lead/extension 100. In certain embodiments, as shown,additional layers 102 lie internal to the covering 74; however, theinvention should not so limited, as the additional layers 102, while notshown as such, can alternatively lie external to the covering 74 just aswell. In certain embodiments, the layers 102 are formed of a braidedmetal sheath or mesh 104 with an optional outer insulation layer 106. Assuch, the additional layers 102 are adapted to enhance the shieldingeffect of the lead/extension 100. In addition, the layers 102 are usefulfor increasing the torsional stiffness of the conductor assembly,thereby aiding the insertion of the lead/extension 100 within thepatient. In certain embodiments, the braided metal sheath or mesh 104can be formed of Ni, Ta, Ti, or superalloy MP35N; however, the inventionshould not be limited to such. Instead, the metal sheath or mesh 104 caninclude any metal or combination of metals or alloys which can be usedto enhance the shielding effect of the lead/extension 100. When used,the outer insulation layer 106 can be formed of silicone, abiocompatible polymer such as polyurethane, or any suitablebiocompatible, non-conducting material known in the art.

FIG. 11 illustrates lead/extension 110. As can be appreciated, thelead/extension 110 is a combination of lead/extension 90 of FIG. 9 andlead/extension 100 of FIG. 10. As such, the lead/extension 110 has thecoating 92 (as described above with respect to FIG. 9) lying external tothe covering 74 and the additional layers 102 (as described above withrespect to FIG. 10) lying either internal (as shown) or external to thecovering 74. As described above, the coating 92 and the additionallayers 102 are used in combination to enhance the shielding effect ofthe lead/extension 110.

FIG. 12 shows lead/extension 120. In certain embodiments, as shown, afurther layer 122 lies internal to the covering 74. The layer 122 isformed of a conductive non-metallic material that is in a wrapped orwoven form. Such conductive non-metallic materials and embodiments inwhich the materials are used for medical device lead shielding aredescribed in more detail in the U.S. patent application entitled“Continuous Conductive Materials for Electromagnetic Shielding”, whichis filed concurrently herewith and incorporated herein in its entirety.In certain embodiments, the conductive non-metallic material can becarbon. The carbon, in certain embodiments, is formed of one or more ofcarbon fiber, carbon nanofiber, and carbon nanotube having one or moreof single and multiple walls. In certain embodiments, the conductivematerial has an outer diameter that is preferably no greater than about20 μm (generally representing minimum diameter of metal wire), and morepreferably, no greater than about 12 μm. The conductive material, incertain embodiments, has a higher skin depth than metals. In certainembodiments, the conductive non-metallic material is covered with ametal coating, thereby enhancing the conductivity of the layer 122. Insummary, the layer 122 is adapted to enhance the shielding effect of thelead/extension 120. While the layer 122 is shown to be internal to thecovering 74, the invention should not be limited to such, as the layer122 could just as well be switched in position with the covering 74 (sothat the further layer 122 lies external to the covering 74) so as tostill enhance the shielding effect of the lead/extension 120.

In certain embodiments, the lead/extension 120 of FIG. 12 may furthercontain one or more of the coating 92 of FIG. 9 and the additionallayers 102 of FIG. 10 to further enhance the shielding effect of thelead/extension 120. If the coating 92 is included on the lead/extension120, in certain embodiments, the coating 92 can lie external to one ormore of the continuous conductive fiber layer 122 and the covering 74.If the additional layers 102 are included on the lead/extension 120, incertain embodiments, the additional layers 102 can lie internal orexternal to the continuous conductive fiber layer 122, and internal orexternal to the covering 74.

It will be appreciated the embodiments of the present invention can takemany forms. The true essence and spirit of these embodiments of theinvention are defined in the appended claims, and it is not intended theembodiment of the invention presented herein should limit the scopethereof.

What is claimed is:
 1. A medical electrical lead, comprising: aconductor assembly having one or more conductors covered by aninsulating layer; and a shield covering adjacent or proximate to atleast a portion of the insulating layer to shield the conductors fromone or more electromagnetic fields, the shield covering comprising apolymer-matrix composite, the polymer-matrix composite including apolymeric resin having discontinuous conductive fillers providedtherein, the discontinuous conductive fillers comprising nano-sizednon-metallic conductive structures, the discontinuous conductive fillersbeing gradiently distributed radially at least within a continuousportion of the polymer-matrix composite with a greater concentration atthe surface of the shield covering and being uniformly distributedcircumferentially and longitudinally at the surface of thepolymer-matrix, the shield covering being circumferentially andlongitudinally continuous over an entire length of the shield covering.2. The medical electrical lead of claim 1, wherein the nano-sizednon-metallic conductive structures are formed of carbon.
 3. The medicalelectrical lead of claim 2, wherein the nano-sized non-metallicconductive structures comprise one or more of carbon nanofibers, carbonfilaments, carbon nanotubes, and carbon nanoflakes.
 4. The medicalelectrical lead of claim 3, wherein the nano-sized non-metallicconductive structures comprise carbon nanofibers.
 5. The medicalelectrical lead of claim 4, wherein the carbon nanofibers have an outerdiameter ranging in size from about 70 nanometers to about 500nanometers.
 6. The medical electrical lead of claim 1, furthercomprising a further layer formed of a conductive non-metallic materialbeing in a wrapped or woven form, the further layer positioned eitherinternal or external to the shield covering, the further layer and theshield covering providing an enhanced shielding effect for the one ormore conductors.
 7. The medical electrical lead of claim 6, wherein theconductive non-metallic material has a higher skin depth than metals. 8.The medical electrical lead of claim 6, wherein the conductivenon-metallic material has an outer diameter that is no greater thanabout 20 μm.
 9. The medical electrical lead of claim 6, wherein theconductive non-metallic material comprises carbon.
 10. The medicalelectrical lead of claim 1, wherein the nano-sized non-metallicconductive structures each has an outer metal coating.
 11. The medicalelectrical lead of claim 10, wherein the outer metal coating prevents anatural tendency of the structures from clumping together.
 12. Themedical electrical lead of claim 10, wherein the outer metal coatingenhances electromagnetic radiation shielding effectiveness of thenano-sized non-metallic conductive structures, the outer metal coatinghaving a thickness of no greater than about 1 μm.
 13. The medicalelectrical lead of claim 1, further comprising a coating formed of oneor more metals, the coating positioned external to the shield covering,the coating and the shield covering providing an enhanced shieldingeffect for the one or more conductors.
 14. The medical electrical leadof claim 1, further comprising one or more additional layers positionedinternal to the shield covering, one of the additional layers comprisinga braided metal sheath or mesh, the one additional layer and the shieldcovering providing an enhanced shielding effect for the one or moreconductors.
 15. The lead of claim 1, wherein the lead is coupled to amedical device.
 16. An implantable medical lead for alleviating theadverse effects of various health ailments of a patient comprising: alead body having a proximal end and a distal end, the distal end; one ormore electrodes or electrical contacts for sensing and/or therapydelivery on the lead body; a conductor assembly having one or moreconductors covered by an insulating layer of the lead body; a shieldcovering coupled to the lead body and comprising a polymer-matrixcomposite, the polymer-matrix composite including a polymeric resinhaving discontinuous conductive fillers provided therein, thediscontinuous conductive fillers comprising nano-sized non-metallicconductive structures, the discontinuous conductive fillers beinggradiently distributed radially at least within a continuous portion ofthe polymer-matrix composite with a greater concentration at the surfaceof the shield covering and being uniformly distributed circumferentiallyand longitudinally at the surface of the polymer-matrix, the shieldcovering being circumferentially and longitudinally continuous over anentire length of the shield covering.
 17. The lead of claim 16, whereinthe lead is coupled to a medical device.
 18. An implantable medical leadfor alleviating the adverse effects of various health ailments of apatient comprising: a lead body having a proximal end and a distal end,the distal end; one or more electrodes or electrical contacts forsensing and/or therapy delivery on the lead body; a conductor assemblyhaving one or more conductors covered by an insulating layer of the leadbody; a shield covering adjacent in a radial direction or proximate inthe radial direction to at least a portion of the insulating layer toshield the conductors from one or more electromagnetic fields, theshield covering comprising a polymer-matrix composite, thepolymer-matrix composite including a polymeric resin havingdiscontinuous conductive fillers provided therein, the discontinuousconductive fillers comprising nano-sized non-metallic conductivestructures, the discontinuous conductive fillers being gradientlydistributed radially at least within a continuous portion of thepolymer-matrix composite with a greater concentration at the surface ofthe shield covering and being uniformly distributed circumferentiallyand longitudinally at the surface of the polymer-matrix, the shieldcovering being circumferentially and longitudinally continuous over anentire length of the shield covering.
 19. The lead of claim 18, whereinthe lead is coupled to a medical device.